Vacuum tube



H. A. SNOW VACUUM TUBE Original Filed June 26. 1930 Patented July 21, 1936 PATENT OFFICE VACUUM TUBE Harold A. Snow, Mountain Lakes, N. J., assgnor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application June ze, 1930, serial No. 464,053

Renewed June 1, 1935 19 Claims. (Cl. Z50-27.5)

VThis invention relates to vacuum tubes of the type including va cathode, a control grid and a plate. lInmy copending application Serial No. 437,225,

`V iiledMarch V19, '1930, vI have described vacuum tubes that are particularly useful as transmission control elements in ,ampler systems. With the known tube constructions, .it is not possible to effect a distortionless transmission, at low amplication, ofstrong signals ifthe tube design is such as to provide high amplification for weak signals. As described in my prior application, distortionless volume or transmissioncontrol for high input voltages may be secured when the elements of l5 the tube `are so constructed and arranged that diierent portions of the electron stream are iniiuenced at dierent rates by the signal voltages applied to the control grid. This particular characteristic may be obtained in several ways, such as, for example, the omission o'f-one or more turnsfrom an end or an intermediate portion of the control grid winding, or by the use of acontrol grid winding of non-uniform pitch.

Tubes of such construction exhibit the Adesired 25 transconductance characteristics which prevent distortion when the operating potentials are so adjusted as to reduce theamplication to a ,small fraction of its maximum value, but there is, in general, a Vrelatively large ow of plate current 30 (direct current) at the .portion of the tube Where the control grid ,is omitted .or is of relatively coarsepitch. i Y- Anobjectof the present invention is to provide a vacuum tube exhibiting .improved trans- 35 mission control characteristics in that no dis- .tortion is introduced even though the input signal voltage 'is raised to values substantially higher than was previously permissible. An additional object isto provide Vavacuum tube capable of 40 Itransmitting high signalvoltages without .sub-

stantial `distortion or withldistortion which is slight, v.but substantially constant over a wide jra'nge'A of signal voltage,gand exhibiting a lower plate current drain -andhigher'plate resistance 45 than :tubes ofthegene'ral type described in jmy copending application. -A further objectV lis to provide, in avacuumtube which at diii'erent portions thereof .exhibits the properties of :a high 'mu and of va'lowtm. tube, :a'shield or grid for re'- 50 ducing the' flow 10i plate fcurrent' at the ylow mu ysectionof the tube'. vMore specifically,- an object :"is zto fprovide, in a l-vacuum V,tube having aV control grid of the general ftype -of a windingfofnon-uniwform pitch, 5a 'low potential ,element :for Yreducing 55 the I:ilow iof plate fcurrent-atgthe portion .of the electron stream which is controlled by the relatively open or coarse section of the grid.

These and ,other objects of the invention will be apparent from the following specification when taken with the accompanying drawing in which: 5

Figs. 1, 2 and 3 are somewhat diagrammatic views, Vin vertical central section, of typical embodiments of the invention, and

' Fig. 4 is a curve sheet` illustrating transconductance characteristics of a tube embodying the 10 invention and other tubesof the same general type. t Y

Theinvention is illustrated in connection with tubes of the known screen grid type employing concentric cylindrical elements, but it is to be understood that the invention is not limited to screen grid tubes .or to the cylindrical type of electrodes.

As shown in Fig. 1, the tube includes within the usual evacuated envelope E, an equipotential cathode C, screen grid SG and a plate P. The control grid CG is constructed in accordance with the invention describedin my copending application and takes the form of a helical winding having a section l of closeand uniform pitch and another more open section 2 of uniform but coarser pitch.

This general type of control grid results in a tube which exhibits the characteristics of a high mu tube when the control grid bias falls within the range of bias voltages corresponding to relatively `high amplification of weak signals. As the signal strength increases, an eiective and practically distortionless control of amplication is aicrdedby that portion of the electron stream that is inuenced by the relatively coarse or more open section 2 of the control` grid CG. As is explained in my copending application, this increase in the effective range of distortionless amplification is accompanied by a high plate current ilow which may be reduced by an appropriate design or location of the other tube elements.

In accordance with the present invention, an additional element or shield S is located adjacent the low mu section of the tube to prevent an excessive flow of plate current when the grid bias is adjusted for high amplification. The shield elements is maintained at a xed or constant Ypotentialmaterially lower than that of the screen grid, 4and vpreferably is connected directly to the 50 cathode by a lead 3. The shield S therefore lowers the positive field from the screen grid, thereby raising the -plate resistance and reducing the yplate current ilow at the coarser section of the control grid. l

The shield S may be supported by the insulator disk 4 and, in tubes of the general type known commercially as the 224 tube, may be constituted by the metal sleeve to which, inthe known commercial tubes, the supporting wiresY 5 of the screen grid SG are attached. The tube as shown in Fig. 1 may therefore .conform tothe standard 224 tube construction so far as concerns the structural details other than the V'spe-3j cial form of control grid winding and the electrical connection of the shield or sleeve Sto the `Y A Y, application. Since,modulationdistortion arises` -whenthe tube is operated upon a sharply curved portion of. the A transconductance characteristic,

cathode instead of the' screen grid.

Except for the addition of a shield element, the tube shown diagrammaticallyin Fig;` 2 mayV be substantially identical with the tube illustrated in Fig. 1 of my copending application, i. e.,fit may conform to standard design of'a high m1`1' 's ':1 "'ee`n,` vgrid tube except for the omission of a plurality of turns from Ythe central section of thelhelicallJ.`

proximately the longitudinal center of that .por-

tion ofV thev control grid from which two turns are Aomitted to form a more openportion. ',The

vshield S is maintained'at a potential lower'than that ofthe plate, orif desired, Hat cathode potential by means ofaconnecting lead 3.`

1t will be apparentthat the Same reduction of plate current may be eiected when .the shield is -located at any4 place ybetween the controlgrid and the accelerating grid SG. As shwnfin Fig.

i 3, the shield S comprises a gridpositionedbelishes va desirable low potential field between the highly positive screen grid SG and thezero po'- tential cathode,. the shield and i the screenV grid constitutingeldfelectrodes which cooperate to produce the desired kind of a` positiveelectrostatic field. The total spacecurrent in a tube Vcontaining* the shield'S maybe, however, about one-half that o f a similantubejnot containing asnield. j '1` i .l 1

As 'will be evident Vfrom the-transconductance curves of Fig. 4, the shield S has the Vfurther advantage of considerably extending-the range of practically distortionless amplification .controlt In Fig; 4, the curve Als thetransconductance curve for acommercialtubeof the 224 type and curve B is a similar curve for atube which differed from standard designby the omissionof two turns from thecenter of the control grid.

The vcurve C is theftransconductance curve `for a tube vwhich includes Ythe low potential shield element adjacent the coarse Asection'of .the control grid.' ItV will be noted that curves B and Csub'- stantially coincide throughouta widel range vof controlV grid bias'voltages, i.`e., u'p to about L65 volts. As theV bias voltage is made more negative to eiect a furtherfreduction in ampliflcation',/the Characteristic for the tube 'm'bodyingth invention, i.l e., curve C, fallsoi less rapidly than l the characteristic, ycurve 28,'i'aftube` does not include the shield element. For each tube,

the possibility lof amplicatlon control is lost when the bias Voltage reaches the value aty which the transconductance curve meets the leakage dition of the shield electrode S increases considerably the useful rangel of transmission control which was a feature of the invention del scribed and claimed in my aforesaid co-pendng it will be evidenti that theV invention alsoin- 1 creases .the range of signalvoltageswhich may be transmittedwithout distortion since the curva-4 ture lof characteristic C is less than that of characteristic B, particularly at those portions of curve ABrwhich are most sharply curved.

i The particular tube Vdesigns which I h avefillus-l trat'ed andthe data obtained in one series of tests are to.v be taken as illustrative of the invention. Y

To one familiar with the design and operation of Vacuum tubes, it willbe"apparent thatjthe in-` 2 vention is vrapplicabletoother types of tubes and to other,physical'arrangements of the electrodesY in tubes of various types.ff v I cla'im; j J f ,1.A vacuum tube including Van equipotentialV3 cs ttliodeggaY plate and a' control grid coextensive with .the cathode andv which exercises diierent rates of control over` 'different portions o the electron stream, anda shieldelementdisposed adjacent said grid for establishing a low poten-. tial j'rleld 'only atthat portionY of the electron jstrearnoveriwhich the saidcontrol grid'exercises a lesser control.

equipo-tential cathode, anode, screen grid' and a 40l control. gridV coextensive with the cathode and which exercises diierent ratesofjfcontrol over dilerent'portionsof the electron stream, a shield element'adjacent saidV cathode for establishing alow Vpotential field at that' portion of the elec# 45 tron, stream over r'whichjtlie' said control gridV exercises ajlesser controL'fand a direct electrical connection from saidshield element to the catli-V ode'.A

and a control grid constituted by a winding of non-uniform pitch, of a shield element-adjacent only 'that portion of said control grid which is ofthe c'oarserpitch'.vr

f 4. -A vacuum tube havingalkcathode, a plate, 5

and a control grid constituted by,a winding `of non-uniform pitch, ot a shieldelement adjacent r` onlythat portioniof said control grid which vis ofthe coarserpitchfanda directV electrical connection from -saidshield element. to thecathode. -5.v A vacuum. tube comprisingv a cylindrical cathode, VafcontrolV grid constituted by -a helical 6.` The invention as set ilorthfin'clai'nll 5, wherein" said shield element Vis located between'said Y vcontrol grid'and said screenfgrld '-1 i if 7. 1'I'he`invention as set forth in laim' am conibinationwith an insulating disk 'lttcd over 'the 7 -3. Ayacuum khavinga 'cathode'gf'a plate, 50

said end of the cathode, said shield element constituting a sleeve supported by said disk and electrically connected to said cathode.

8. An electron discharge tube including a cathode, anode, screen grid and a control grid which exercises different rates of control over diierent portions of the electron stream, and a shield element adjacent said cathode for establishing a low potential eld at that portion of the electron stream over which the said control grid exercises a lesser control, said element being connected to said cathode, said shield element being disposed between said screen grid and cathode.

9. An electron discharge tube including a cathode, anode, screen grid and a control grid which exercises different rates of control over diierent portions of the electron stream, and a shield element adjacent said cathode for establishing a low potential eld at that portion of the electron stream over which the said control grid exercises a lesser control, and a direct electrical connection from said shield element to the cathode.

10. An electron discharge tube including a cathode, anode, screen grid and a control grid which exercises different rates of control over different portions of the electron stream, and a shield element adjacent said cathode for establishing a low potential field at that portion of the electron stream over which the said control grid exercises a lesser control, said element being connected to said cathode, said control grid being provided with fine and coarse spaces, said shield surrounding said coarse space and being disposed between said screen and control grid.

11. An electron discharge tube including a cathode, anode, screen grid and a control grid which exercises different rates of control over different portions of the electron stream, and a shield element adjacent said cathode for establishing a low potential eld at that portion of the electron stream over which the said control grid exercises a lesser control, said element being connected to said cathode, said control grid being provided with an open portion at substantially its longitudinal center, and said shield surrounding said open portion.

12. An electron discharge tube including a cathode, anode, screen grid and a control grid which exercises dierent rates of control over dilerent portions of the electron stream, and a shield element adjacent said cathode for establishing a low potential eld at that portion of the electron stream over which the said control grid exercises a lesser control, said element being connected to said cathode, said control grid being provided with a more open portion adjacent one end of said cathode, said shield surrounding said cathode and being disposed between the control grid and cathode adjacent said open portion.

13. A vacuum tube comprising an electron emitting element, a screen grid, a plate, and a control grid which exercises diierent rates of control over diierent portions of the electron stream, and means for reducing the flow of plate current at that portion of the electron stream over which the said control grid exercises a lesser control comprising a shield element disposed adjacent that portion of the control grid which exercises the lesser control.

14. A vacuum tube comprising a control grid constituted by a winding of non-uniform pitch, a cathode, a plate, a screen grid, and means for reducing the now of plate current at that portion where the control grid is of coarser pitch, without substantially altering the transconductance oi the tube, comprising a shield element disposed adjacent the coarser-pitched portion of the control grid.

15. An electron discharge device comprising a cathode, a. control grid which exercises different rates of control over different portions of the electron stream from said cathode, a plate and a shield electrode adjacent only that portion of said grid which exercises a lesser rate of control than elsewhere, and means for maintaining said shield electrode at a xed potential diierent than the potential of said grid.

16. An electron discharge device comprising a cathode, a control grid coextensive with and surrounding said cathode and having a portion which is more open and exercises a lesser rate of control over the electron stream from said cathode than the remainder of said grid, a plate, and a shield electrode cooperating only with the more open portion of said grid to exercise on the electron stream from said cathode through the more open portion of said grid a rate of control which differs from the rate of control exercised by the other portions of said grid.

17. An electron discharge device comprising a cathode, an anode, a control grid coextensive with said cathode and interposed between said cathode and said anode and having a portion more open than other portions and exercising a lesser rate of control on the electron stream from said cathode to said anode, and an electrode of fixed potential adjacent only the more open portion of said grid for making the positive eld of said anode weaker at said more open portion of said grid than at other portions.

18. An electron discharge device comprising a cathode, a control grid coextensive with and surrounding said cathode and having a portion more open than other portions and exercising a lesser rate of control on the electron stream from said cathode, a positive electrode surrounding said control grid, an auxiliary cold electrode disposed adjacent only said more open portion of said control grid, and means for establishing on said auxiliary electrode a xed potential different than the potential of said positive electrode.

19. An electron discharge device comprising a cathode, a high potential plate surrounding said cathode, a control grid coextensive with and interposed between said cathode and said anode and having a portion more open than other portions and exercising a lesser rate of control on the electron stream from said cathode to said plate, and an element which is disposed adjacent only the more open portion of said control grid and which at a fixed potential lower than the potential of said anode reduces the flow of plate current at said more open portion.

HAROLD A. SNOW. 

